With regards to iPhone user App purchases anyway... anything else is up for debate.
A story on TUAW says that the average iPhone user has spent about $80 on apps with an average of 65 apps per user... working out to an average spent of about $1.56 per app.
Me? According to my email receipts from Apple, I've spent roughly $198 on about 104 apps (before tax). That works out to about $1.90 per app for me. I've had a lot of free ones (obviously) and a lot of paid ones. I don't know exactly how many of each, but I've spent up to $13 or more on some apps (Dynolicious is one that comes to mind). But usually just $.99 or maybe $5.
One of the important things to note here is that this goes back to 2007 or 2008 when I first got the iPod Touch. So for a full year or two, I don't think that's too bad. Works out to just a couple $$ per month, if that.